This invention relates to a roller cutter drill bit used to drill bores in the earth and more particularly to a so-called mining roller cutter drill bit of the type adapted to be detachably secured to a drill string for rotating the bit and delivering drilling fluid under pressure to the bit for removing cuttings from the bore and for cooling the bit.
This invention involves an improvement over roller cutter drill bits of the type such as shown for example in U.S. Pat. No. 3,685,601, comprising a bit body having means at its upper end adapted to be detachably secured to a drill string, a plurality of spaced-apart legs at its lower end, each having a bearing journal, and passaging in the bit body extending down to each of the journals for flow of the fluid under pressure to the journals. The bit further includes a plurality of roller cutters, and bearing means between each roller cutter and respective journal for rotatably mounting the roller cutter on the journal. With the bit in the drill bore and with fluid under pressure being supplied to the drill string in sufficient quantity, the fluid flows through the passaging in the body past the bearing means in the bearing cavity and exits the bit via the annular opening between the roller cutter and the journal for cooling and cleaning the bearing means. Upon exiting the bit body, the fluid under pressure then flows up the annulus between the drill string and the well bore carrying cuttings away from the bore bottom. The cuttings consist of dust and relatively small particles of the formation broken from the well bore bottom by the roller cutters.
While the conventional mining drill bit has been generally satisfactory, problems have arisen when the flow of drilling fluid pressure has been insufficient to keep the bearing means clean of cuttings. Typically this happens when the drill bore has been drilled to the desired depth and the drill bit is withdrawn from the bore with the supply of drilling fluid shut off. Entry of cuttings into the bearings may also happen when in drilling, the drill bit unexpectedly drills through a stratem of hard formation into soft formation material. Because the rate of penetration and thus the quantity of cuttings generated in drilling soft formations is greater than in drilling hard formations, the relatively low flow rate of drilling fluid satisfactory for hard formation drilling may be insufficient to keep cuttings out of the bearing cavities during soft formation drilling. In either of these instances of insufficient flow of drilling fluid, the result is that bore cuttings enter the bearing cavity and foul the bearing means thereby rendering the drill bit unusable for further drilling operations unless these cuttings are somehow removed from the bit. Typically, the supplying of drilling fluid in an increased quantity is not adequate to remove the cuttings, particularly once the drill bit is removed from the drill bore and allowed to cool, with the cuttings thus becoming "set" in the bearing cavity.